1. Field of the Invention
The invention relates to electric melting techniques, and more specifically those in which energy is dissipated in the molten mass by Joule effect, by means of plunging electrodes.
2. Discussion of the Background
For a long time, glass production installations which operate on large quantities have been provided with melting furnaces which are supplied with fossil fuel, such as fuel oil or gas. This is the case in particular for large capacity continuous production installations which for example provide flat glass or bottle glass. When electrical energy is in these large furnaces, it is essentially as a local booster, in order to maintain the temperature of the glass in the least hot areas, or outside the furnace along the path of the glass towards its place of transformation, or in order to develop specific convection movements, to assist homogenization, refining or transport of the molten material.
True electric melting was used firstly in small units in which considerable flexibility in the conditions of use appeared to be necessary. Fluctuations of energy costs and gradual mastery of certain technological problems have led more recently to the development of major production units in which all of the melting process, with the exception of commissioning, takes place using electric energy. This development requires the solution of extremely delicate technological problems.
Thus, in particular, in order to avoid oxidation of the electrodes at the surface of the melting bath, it has been proposed to immerse the electrodes completely. This is the solution used for example in French patent application FR-A-2 552 073. In this document, the electrodes are disposed vertically in the bath, such that they project from the hearth of the furnace. In other embodiments, electrodes also pass through the lateral walls of the furnace.
Irrespective of the advantages it provides relative to corrosion problems, immersion of the electrodes also permits convenient, regular supply of the surface of the bath with a composition of raw-materials. Formation of a relatively thick layer of composition to be melted, floating on the molten bath, is in fact advantageous for several reasons. In contact with the melting bath, it provides the permanent reserve of material necessary for continuous operation. It also protects the melting bath against substantial loss of heat by convection in contact with the atmosphere, and in particular by radiation.
Although furnaces of the type described in the aforementioned document have very important industrial applications, they do not necessarily provide the best solution to all requirements encountered in practice. For example, in some cases, and with the obvious purpose of limiting investment costs, it is desirable to transform installations which operate with burners by maintaining as many as possible of the existing components, and in particular the refractory materials which constitute the pouring basin. Transformation of this type is not possible when the electrodes are to be installed in the hearth or in the lateral walls of the furnace.
Furnaces of which the electrodes are immersed, have limited possibilities for adjusting the electrodes. Although they provide entirely satisfactory performance for specific operating conditions, they are less suitable for frequent and/or substantial modifications of these operating conditions.
In addition, even though the technology of immersed electrodes has now been well-mastered, and a long service life of the electrodes which is comparable with that of the refractory units can be envisaged, the risk of premature deterioration of one of a plurality of electrodes, thus affecting satisfactory operation, cannot be altogether eliminated.
Another solution which is described in particular in French patent application 2 599 734, consists of plunging electrodes through the free surface of the bath of molten material. This technique has a given number of advantages. Firstly, it obviously prevents the difficulties associated with the passage of these electrodes through the refractory unit, and also problems of replacement of worn electrodes, sealing or wear of the refractory units, in particular owing to a high temperature which assists attack of the refractory units, and powerful convection currents which develop in contact with the latter.
The technique of plunging electrodes localises the hottest areas in the upper part of the molten bath, and thus attenuates these problems.
In addition, this technique permits modification of the depth of immersion of the electrodes, and thus the temperature gradient. This allows the draught of the furnace to be modified without changing the temperature of the hearth, and thus the temperature of the glass at the furnace outlet.
Additionally, experience has shown that this technique has a very satisfactory thermal output, and provides good quality of the molten material.
Plunging electrodes are conventionally attached to supports which overhang the melting basin from the sides of the latter. Application FR-A-2 599 734 describes a support of this type which consists of an arm which comprises ducts for circulation of the cooling fluid, and an electric cable for supplying the electrode and the electrode support.
In normal operation, a layer of composition deposited on the surface of the melting bath, which constitutes protection against thermal losses, prevents the temperature of the arms which overhang the melting bath from becoming too high.
On the other hand during a waiting period, in which the layer which protects the raw materials is either very thin or lacking, the temperature of the arm becomes very high, and gives rise to deterioration of the electric supply system.
In order to eliminate this disadvantage, a conventional solution consists of raising the plunging electrodes during a waiting period, and maintaining a sufficient temperature in the bath by means of immersed electrodes, which are usually disposed on the walls. This technique is effective, but there are again problems associated with the immersed electrodes, although in the present case they operate at lower voltages, since they are only maintaining the temperature of the already molten bath. Moreover, immersed electrodes of this type require additional investment costs.
Another solution proposed, which is described in particular in U.S. Pat. No. 4,965,812, consists of using an electrode support which consists substantially of a current conductor water-Jacket type cooling system. The supply system is then cooled continually, and thus protected against the temperature increase which occurs during a waiting period. On the other hand this type of installation requires a protection device, since the power to the electrode support is maintained permanently switched on.
A device of this type usually consists of a grid which prevents the operators from gaining access to the furnace. However specific steps which require the presence of an operator in the vicinity of the bath, and thus of the electrode supports, put the operator in danger.